The present invention relates to a smoke composition and method of making the same; more particularly to a red-phosporusbased composition, which may be used with a pyrotechnic tube as a marker in search and rescue operations to emit both flame and smoke.
Very often, in the course of search and rescue operations by military and civilian agencies, the need arises to distinctly mark a particular point on water. The deployed marker must be highly visible night or day so that a combination marker, emitting both flame and smoke, is necessary. The marking device, which can be dropped into water by aircraft, must float in water. It must give as large a volumne of smoke and flame for as long as possible.
At the present time, the marker most commonly used by many organizations for marine marking is one which produces a flame and smoke by burning a jet of phosphorus vapor which comes out of a small opening at the top of the marker and spontaneously inflames when it contacts air. Upon functioning, the marker emits a bright yellow flame and a large quantity of white smoke for a period of time (e.g. 15 to 18 minutes). Such marker device is considered robust, reliable and relatively compact.
The phosphorus vapor for such a marker is generated by the vaporization of red phosphorus (hereinafter RP) from a pyrotechnic composition which has been pressed as a candle inside a pyrotechnic tube. An example of such a composition is:
______________________________________ INGREDIENTS TYPICAL FORMULATION ______________________________________ red phosphorus (RP) 51.5% manganese dioxide, MnO.sub.2 35.1% magnesium powder, Mg 7.2% zinc oxide, ZnO 3.1% linseed oil (double boiled) 3.1% ______________________________________
The main reaction in such a conventional marker is between manganese dioxide and magnesium. This reaction produces the heat required to vaporize the RP. The linseed oil functions both as a lubricant during pressing of the smoke candle and as a binder, holding the ingredients together after pressing the composition. The zinc oxide is added as a stabilizer, mainly to neutralize any acids that are formed as the composition ages.
Manufacture of conventional marker candles is accomplished according to well-defined and proven techniques. After all of the ingredients have been intimately mixed, the composition is pressed into cylindrical, laminated paper tubes to form the candles for the markers. The pressing is done at 9900 psi in 3 increments with a dwell time of 3-10 seconds for each increment. The pressing operation must be performed no later than 8 hours after mixing the composition, otherwise the linseed oil will dry and become useless as a binder.
Nominal specifications for such markers call for 744.+-.40 g of composition pressed into a laminated paper tube 29.9 cm long, with an internal diameter of 4.4 cm. Typical composition length is 25-26 cm, which leaves several centimeters for a starter composition. The linear burn rate for such a marker candle is approximately 1.3 cm per minute.
Although such a composition functions quite reliably as a marine marker, several problems and shortcomings exist with its production and performance. The chief problem is the fire hazard that exists during production of the composition. RP is very prone to ignition by the friction present during mixing. The risk of ignition is further augmented by the presence of oxidizer (manganese dioxide), so that fires are common during production of this compositon. In addition, the pressed compositon, once fully dried, is quite brittle and tends to crack. The hard, jagged surfaces at the crack can ignite very easily if allowed to rub against each other.
Performance is hampered by the low loading of RP in the composition, caused by the use of a low efficiency oxidizer, manganese dioxide (MnO.sub.2). Because of its low active-oxygen-content, the oxidizer must account for over one-third of the weight of the composition--room that could otherwise be filled with RP. Magnesium metal is the fuel in the composition, which, upon reacting with manganese dioxide provides the heat necessary to vaporize the RP. Although, magnesium is an excellent high-energy fuel, it is prone to corrosion if there is any moisture present. It reacts with the small amounts of water always present in the composition to give hydrogen, as well as promoting the formation of phosphine (PH.sub.3) gas from the RP. In addition, corroded magnesium is useless as a fuel.
Several processing and performance problems arise from the use of linseed oil as the binder. Its main disadvantage, from a processing standpoint, is the time limit it imposes between the mixing and pressing operations. The linseed oil begins to dry as soon as it is incorporated in the composition and exposed to air, with the result that the composition must be pressed within 8 hours, the time the oil takes to dry.
Performance is affected by the relatively poor binding properties of the oil. As mentioned previously, the pressed composition has poor mechanical properties and is easily fractured by stress, such as that caused by the marker impacting in the water or during rough handling. Such fractures not only increase the risk of spontaneous ignitions but may result in reaction- propagation-failure during regular functioning of the marker.
Finally, there are problems with the use of zinc oxide as a stabilizer, including the poor acid-absorbing capability of that compound and the fact that it is somewhat toxic.
It is an objective of the present invention to provide an improved composition to be used in producing marine markers (or indeed, producing markers for other purposes) which is not the subject to the drawbacks of the conventional composition, as previously described. Itis a further objective of the present invention to provide a composition which has an increased smoke-producing efficiency.